When using this accessory lens adapter with the spotting scopes, it alters the focal length. When using it with the 65 and 85mm spotting scopes, the focal length is 750-1800mm. When using it with the 95mm spotting scope, the focal length is 900-2100mm.

This adapter ring is threaded on the outside with T-threads and inside with c-mount threads. When used in place of the nosepiece on an ST-402/1603/3200 model camera, c-mount lenses can be used for wide field imaging.